1. Field of the Invention
The invention relates generally to the field of photography, and particularly to a film threading apparatus in a photographic camera.
2. Description of the Prior Art
In conventional 35 mm film manufacturers' cassettes, such as manufactured by Eastman Kodak Company and Fuji Photo Film Co. Ltd., the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading portion of the filmstrip approximately 21/3 inches long, commonly referred to as a "film leader", protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is initially rotated in an unwinding direction, the film roll inside the shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the fogged leader portion protruding from the slit will remain stationary. The film roll can expand radially until a non-slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Consequently, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to wind the protruding leader portion onto a film take-up drum in order to draw the filmstrip out of the shell. Since the protruding leader portion is usually laid over the exPosure (back frame) opening in the camera during film loading, it is not uncommon to retract the film pressure plate (for holding each successive film frame flat at the exposure opening) preparatory to winding the leader portion onto the take-up drum. See U.S. Pat. Nos. 4,525,051; 4,505,562; 3,563,144; 2,753,777; and 2,336,279.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance the filmstrip out of the cassette shell by rotating the film spool in the unwinding direction. The leader portion normally does not protrude from the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3, 1984, there is disclosed a film cassette wherein the outermost convolution of the film roll wound on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner curved wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the leading end of the filmstrip is slightly tapered along one longitudinal edge purportedly to allow it to freely extend from between the circumferential lips and rest against the shell wall. During initial unwinding rotation of the film spool, the leading end of the filmstrip is advanced along the shell wall until it reaches an entry to a film passageway in the cassette shell. Then, it is advanced into and through the film passageway to the outside of the cassette shell. The passageway has a width that is slightly less than the width of the filmstrip, thus resulting in the filmstrip being transversely bowed as it is uncoiled from the film spool, and thereby facilitating movement of the film edges under the circumferential lips of the respective flanges.
While the film cassette disclosed in U.S. Pat. No. 4,423,943 includes a non-protruding film leader which is automatically advanced to the outside of the cassette shell when the film spool is rotated in the unwinding direction, the film cassette is intended to be loaded in a camera only after the film leader is advanced to protrude from the cassette shell. In the patent, it is suggested that one manually rotate the film spool relative to the cassette shell until the leader portion can be manually grasped and attached to a film advancing device in the camera.